Numerical investigation on transient aero-thermal characteristics of a labyrinth regulating valve for nuclear power plant

Due to cavitation protection, labyrinth regulating valves are widely employed in the steam turbine bypass system of nuclear-power plants. Transient opening and closing behavior, including the flow and thermal characteristics, has an important impact on the performance of the bypass system. In order to acquire a deep insight into the transient characteristics of the labyrinth valve, a numerical simulation of the transient opening and closing process of a labyrinth regulating valve was carried out based on the moving mesh technique, multiple domains and valve spool motion using user define function (UDF) in the software. Firstly, the flow capacity of the simulated valve was validated against the experimental results. The flow behavior, including the local flow resistance and pressure-reducing ability in three different labyrinth passages, was compared and discussed. Finally, the transient aerodynamic and thermal behavior was investigated in detail by field synergy theory, and the transient energy loss was also quantified based on the entropy production rate. The distribution of local pressure gradient and normalized pressure difference, caused by backward-facing step flow, indicate that the third disc labyrinth at the bottom of spool has the most significant pressure-reduction ability than the other two discs. The pressure-reducing performance of the disks is determined by the “cells” number and circumferential width of the channel inlet. Moreover, the pressure-reducing ability of labyrinth passages becomes weaker with the increase of valve opening. In the opening process, the distribution of entropy production rate caused by heat transfer (EPR₂) implies that heat transfer loss increases firstly and then decreases with the rise of opening, which is similar to the dissipation loss and flow resistance loss. Furthermore, the peak point of EPR₂ during the opening process is 12.9% greater than that during the closing process.